1. How do people get infected with West Nile virus (WNV)?

People get West Nile virus from the bite of a mosquito that has fed on an infected bird. Mosquitoes can't transmit the virus from one person to another.

It's also possible to get West Nile virus from a blood transfusion or an organ transplant from an infected donor. Since all blood donated in the U.S. is tested for the virus, the blood supply in this country is considered safe.

2. What are the symptoms of West Nile virus infection?

Four out of five people infected with West Nile virus don't have any symptoms. A few of these people may develop lingering infections that might cause problems years later. But most will become immune to the virus and never get a West Nile illness.

One in five infections results in West Nile fever. Once considered a relatively mild illness, the CDC now says people with West Nile fever can be quite ill for quite a long time.

In one in every 150 infections, the virus gets into the brain (encephalitis) or into the tissues covering the brain and spinal cord (meningitis). This is what the CDC calls "neuroinvasive" West Nile disease.

About 10% of people who get West Nile encephalitis or West Nile meningitis die. Most people with neuroinvasive disease have either encephalitis or both encephalitis and meningitis. Some 25% to 35% get meningitis only.

Neurological symptoms, including paralysis of one or more limbs or palsy, may occur.

Tremors and movement problems sometimes occur.

Some people with West Nile infection become weak or paralyzed in one or more limbs. Two-thirds of these people are left with permanent weakness or paralysis. It's not yet clear what percentage of people with neuroinvasive West Nile disease get this syndrome, known as West Nile poliomyelitis.

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3. How is West Nile virus treated?

There is no specific treatment for West Nile virus infection. More severe cases require intensive hospital care.

4. What are the risks of West Nile virus infection for pregnant women?

There have been a few miscarriages among women infected during pregnancy, but it's not clear whether the virus played a role. Most women known to have been infected with West Nile virus during pregnancy have given birth to normal, uninfected babies.

While pregnant women appear not to be at particularly high risk from West Nile infection, uncertainties remain. The CDC advises pregnant and nursing women to take special care to avoid mosquito bites.

5. Should I continue breastfeeding if I have symptoms of West Nile virus?

At least one woman appears to have passed West Nile virus to her child during breastfeeding. This appears to happen only rarely.

The benefits of breastfeeding far outweigh the potential risk of West Nile infection to an infant.

6. What are the chances of dying from West Nile virus infection?

The odds of getting the most severe forms of West Nile disease are about one in 150. The overall death rate in severe disease is about 10%. That makes the overall odds of dying from a West Nile infection about one in 1,500.

These odds aren't the same for everyone. People over age 50, especially those who are elderly, are more likely to suffer severe consequences from West Nile infection.

7. How does West Nile virus actually cause severe illness and death in humans?

West Nile virus causes severe disease when it crosses the blood/brain barrier and infects the brain and spinal cord. The virus disturbs normal brain function -- including disruption of the nerve impulses needed for breathing -- which can be fatal.

8. Should people avoid donating blood or getting blood transfusions or organ transplants?

Blood is lifesaving and often in short supply. Donating blood is safe, and we encourage blood donation now and in the future. Approximately 4.5 million persons receive blood or blood products annually. Although persons needing blood transfusions or organ transplants should be aware of the risk for WNV infection, the benefits of receiving needed transfusions or transplants outweigh the potential risk for WNV infection.

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9. How do health care providers test for West Nile virus?

If your doctor suspects you have a West Nile virus infection, he or she will send a sample of your blood to a lab for tests. The most common tests look for antibodies against the virus, showing that you've recently been infected.

If you have symptoms of more severe disease, your doctor may perform a spinal tap to collect spinal fluid. As with blood, the spinal fluid sample is sent to a lab for tests.

10. Who is at risk for getting West Nile virus infection?

Mosquitoes carrying West Nile virus are found in all of the lower 48 states. Mosquitoes are most plentiful in late summer. West Nile season usually peaks in August and September.

People who spend a lot of time outdoors are at highest risk of infection. The mosquitoes that transmit West Nile virus are most active at dawn and dusk. Being outdoors during those hours increases the risk of infection.

People over age 50 are more likely to develop symptoms of West Nile disease, but it's not clear whether older people are more susceptible to infection.

Infants are not at higher risk of West Nile virus infection. And pregnancy does not increase a woman's risk of infection.

11. Can you get West Nile encephalitis from another person?

No. West Nile encephalitis is not transmitted from person to person. For example, you cannot get West Nile virus from touching or kissing a person who has the disease or from a health care worker who has treated someone with the disease.

12. Where did West Nile virus come from?

West Nile virus was first detected in the U.S. in New York City in 1999. It is not known where the U.S. virus originated but it is most closely related to strains found in the Middle East.

13. Is West Nile virus seasonal in its occurrence?

In the U.S., West Nile virus cases occur primarily in the late summer or early fall. In southern climates, where temperatures are milder, West Nile virus can be transmitted year-round.

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14. Is there a vaccine available to protect humans from West Nile virus?

No. However, vaccines have been made against similar viruses, so a West Nile vaccine appears possible.

15. If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.

16. If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?

It is assumed that immunity will be lifelong; however, it may wane in later years.

17. If my dog/cat becomes infected with West Nile virus, does it pose a risk to the health of my family or other animals?

While cats and dogs can get West Nile infections, they cannot spread the virus to people.